1. Field of the Invention
The invention relates to a method of mounting a microcircuit, previously mounted on a connecting board, inside a cavity prepared for this purpose in a card forming a support.
The invention relates more particularly to coating the microcircuit with a hardenable material of the polymerizable resin kind by a process that very greatly increases the resistance of the card to bending.
The invention also relates to any microcircuit card, commonly referred to as a xe2x80x9csmart cardxe2x80x9d, incorporating a structural feature resulting from use of the above method, as indicated hereinafter.
2. Description of the Prior Art
One of the most widely used methods of mounting a microcircuit in the cavity of a card entails first forming a module consisting of a connecting board of the printed circuit type, fixing the microcircuit to the board and then gluing the module into a blind cavity formed in the card. The expression xe2x80x9cblind cavityxe2x80x9d refers to a recess open on one face of the card and whose depth is less than the thickness of the card. The cavity therefore has a bottom consisting of a thinner part of the card. The microcircuit is usually connected by soldering wires to external contacts of the printed circuit. A protective resin is then deposited on the microcircuit before fitting the module into the cavity.
An ISO standard characterizes the resistance of the card to bending stresses. Mechanical bending cycles are applied to the card which must not break mechanically or electrically following a particular number of such bending cycles.
The mounting technique referred to above has the drawback of concentrating a major part of the mechanical bending stresses on the microcircuit itself. Attempts have been made to shift the extreme stress areas to areas external to the module. For example, weakened areas have been created external to the module by locally reducing the thickness of the support or applying reinforcing structures to the module. These methods have the following drawbacks:
The plastics material card is mechanically weakened in given areas, which can lead to the plastics material breaking in those areas.
The creation of weakened areas changes the visual appearance of the card with poor esthetic results. The structure is complex.
Another technique is described in European patent 0 519 564. In this technique, when fitting the module, the microcircuit is inserted into the cavity in the support after depositing in the cavity a protective resin that coats the microcircuit and fills substantially all of the cavity. This stiffens the area of the card containing the microcircuit, which protects it to some degree from mechanical bending stresses.
An annular rib machined into the bottom of the cavity confines the resin deposited before gluing to the center of the cavity. The rib is preferably discontinuous to enable excess resin to flow out towards the edges of the cavity when the microcircuit is pressed into the drop of resin. The object of this prior art technique is to fill the greater part of the cavity whilst preventing the resin overflowing when the module is fitted.
However, the rigid block consisting of the microcircuit and the resin transfers mechanical stresses to the perimeter of the cavity, to be more precise to the vicinity of the perimeter of the bottom of the cavity. During repeated bending cycles, the card tends to crack in this thin area.
The presence of the annular rib slows down machining and prevents the mounting of large integrated circuits in a cavity of given dimensions, other things being equal. The rib can also come into contact with the wires connecting the microcircuits to the connection areas, which can break the electrical connections to the microcircuit.
The invention proposes an improvement to the above technique for coating the microcircuit which, among other things, very significantly reduces the bending stresses in the vicinity of the periphery of the bottom of the cavity and eliminates the confinement rib.
A first aspect of the invention reduces the stresses transmitted to the periphery of the bottom of the cavity to a surprising degree merely by freeing this area of the resin for fixing and coating the microcircuit.
To be more precise, the invention relates to a method of mounting a microcircuit in a cavity in a card forming a support, in which method a module consisting of a connecting board carrying the microcircuit is formed and fixed to the card so that the microcircuit is housed in the cavity, the fixing of the module including coating the microcircuit with a coating material deposited on the bottom of the cavity and adhering thereto, for example a resin that is still liquid, and hardening the coating material, and a compressible annular area is formed in the cavity, completely surrounds the coating material and extends in part over a continuous peripheral area of the bottom of the cavity.
In a preferred embodiment of the invention at least part of the compressible annular area is formed by forming a ring of air in the cavity all around the coating material.
A thixotropic resin is preferably used. Thixotropic viscous substances lose some of their viscosity on agitation or rapid shear movement. At rest, the substance reverts to its initial viscosity. By an appropriate choice of its rest viscosity, a thixotropic resin in the liquid state, i.e. before it hardens, can be deposited on the bottom of the cavity, for example in the form of drops or beads, without spreading. When the resin is deposited, its viscosity decreases rapidly as it is expelled through the metering nozzle. Once deposited on the bottom of the cavity, it reverts to its initial high viscosity, which provides better control over the required distribution of the resin, and so the required ring of air is certain to be formed. When the resin has been spread after mounting the module, it retains its original viscosity and the shape of the ring of air obtained does not change during polymerization.
It is advantageous to apply localized surface treatment to the central area of the bottom of the cavity before depositing the resin. The treatment is preferably applied by means of a laser beam. This increases the surface energy of the central area and the bottom of the cavity then has two areas with different surface states substantially concentric with, and both extending beyond, the location of the microcircuit (at the center of the cavity). The spreading of the resin is then dependent on the shapes, dimensions and surface states of these areas.
More particularly, if the boundary between the treated central area and the untreated peripheral area, which has a lower surface energy, is substantially coincident with the required inside edge of the ring of air, the resulting configuration stops the resin spreading during mounting of the module. This confinement of the resin to the center of the cavity guarantees that the required ring of air is formed correctly. The behavior of the resin due to different surface states of concentric areas of the support is described in detail in U.S. patent application Ser. No. 09/563,910 now U.S. Pat. No. 6,372,541. filed conjointly with the present application.
Treating the surface of the central area of the bottom of the cavity also improves the adhesion of the resin. It must be possible to polymerize the resin under conditions that the other components can withstand. For example, the polymerization temperature must be less than the temperature at which the plastics material softens, i.e. from 50xc2x0 C. to 80xc2x0 C., depending on the material.
For example, good results have been obtained with an insulative epoxy resin which typically has a viscosity of the order of 2,900 cps (centipoises) and a thixotropic index (the ratio of the viscosity at 5 rpm to that at 50 rpm) of 2.8. Polymerization is obtained in 48 hours at room temperature. The Shore D hardness of the resin after polymerization is from 70 to 80. The plastics material constituting the card body in which the cavity is defined is advantageously a material of the polybutylene-terephthalate type. By combining all the above features it is possible to obtain a card able to resist more than 80,000 mechanical cycles in the direction of the shorter side of the card, as specified in the relevant ISO standard.
Other materials can be used for the card body. If polybutylene-terephthalate is used, as mentioned above, which is characterized by a low surface energy, the surface treatment of the bottom of the cavity is particularly recommended to achieve good adhesion of the resin. A significant improvement in adhesion has been obtained by surface treatment such as flame, corona or plasma treatment (with Ar or Ar/O2). However, as mentioned above, the laser beam treatment has the additional advantage of making it easier to localize the surface treatment at the center of the bottom of the cavity, which provides the benefit of the phenomenon described hereinabove at the time of spreading the resin, and therefore better control over the formation of the ring of air.
In one variant, the compressible annular area can be obtained at least in part by placing in said cavity a ring of elastomer material extending all around the coating material. To be more precise, the ring of elastomer material can be deposited along the perimeter of the bottom of the cavity before applying the resin so that the resin is kept away from the perimeter of the bottom of the cavity, regardless of how it spreads. Forming a ring of air all around the coating material is the preferred solution, but it is also feasible to combine the two techniques, said compressible annular area being obtained by providing side-by-side in the cavity a ring of air and a ring of elastomer material.
The distribution of the resin over the bottom of the cavity is also important. The configuration of the deposit of resin is preferably determined by the shape of the cavity, in particular dictated by the shape of the perimeter of the bottom of the cavity and the geometry of the microcircuit concerned.
For example, if the bottom of the cavity has an at least approximately rectangular or oval contour, several drops of resin are preferably deposited to define a configuration that is substantially centered relative to the bottom of the cavity and approximately the same shape as the cavity so that when the microcircuit is placed in the cavity the drops intermingle and spread to form a ring of air along the walls of the cavity.
For example, for a rectangular cavity, it is sufficient to deposit on its bottom wall two drops offset in the lengthwise direction of said cavity along a median line thereof.
Another possibility is to deposit a plurality of small drops, i.e. in practice a dozen or more, defining an approximately rectangular or substantially oval configuration, elongate in the lengthwise direction of the respectively rectangular or oval cavity. To allow for the thickness of the microcircuit, the quantity of resin deposited at the center of the configuration is preferably less than that deposited on the other parts of the bottom.
The invention also relates to a microcircuit card including a cavity housing a module consisting of a connecting board carrying a microcircuit and a compressible annular area formed in the cavity, completely surrounding a material for coating the microcircuit between the module and the bottom of the cavity, and extending in part over a continuous peripheral area of the bottom of the cavity.
The invention will be better understood and other advantages of the invention will appear more clearly in the light of the following description of a method of mounting a microcircuit in a cavity in a card forming a support, which description is given by way of example only and with reference to the accompanying diagrammatic drawings.